The long-term objective of this project is a thorough understanding of the behavioral and neural mechanisms of sound localization. The present application will continue our long-standing electrophysiological and anatomical studies of the brainstem circuits thought to underlie sound localization and incorporates our new behavioral techniques to combine physiological recording and behavioral analysis in awake cats. We have three general aims. Aim I continues our studies seeking physiological correlates of the well-known psychophysical phenomenon known as the precedence effect, but now we will record from the interior colliculus of behaving cats. These experiments will explore the effect of behavioral states on the responses to stimuli that are known to evoke the precedence effect in human subjects as well as to resolve differences reported in awake and anesthetized animals. Aim II will study an important source of inhibitory input to the inferior colliculus, the dorsal nucleus of the lateral lemniscus (DNLL). We will study the physiological response properties of the DNLL, focussing on their binaural response and sensitivity to interaural time differences, use intracellular recording and staining of single cells to study structure/function relations, and inactivate the DNLL while recording from cells in the contralateral inferior colliculus to study the function of this bilateral inhibitory input. Aim III will use the virtual acoustic space technique to study responses of cells in the lateral superior olive (LSO) with particular attention to exploring the sensitivity of cells to spectral cues. These studies will provide important information on the neural mechanisms underlying sound localization. Spatial hearing is an important basic function of the auditory system: defects in binaural function in human patients can lead to difficulty in understanding conversations in a noisy room, which is perhaps the most common complaint of the hearing-impaired and can lead to severe social withdrawal.